buttcrack. Cipher Wiki

Gronsfeld

From the Buttcrack Cipher Wiki — the free field guide to classical ciphers

Gronsfeld — Vigenere with a numeric key, so each column has ten possible shifts instead of twenty-six.

Vigenere restricted to a digit key, so each column has only 10 possible shifts.

A worked example§

Encrypting a sample with the cipher itself, at build time:

MEET ME BY THE OLD CLOCK TOWER AT DAWN

Key: 31415

produces the ciphertext

PFIU RH CC UMH PPE HOPGL YRXIS FW EEXS

This example is generated by running the cipher when the page is built, and the round trip is checked, so it always matches what the solver does.

How it is broken§

Easier than Vigenere: the reduced alphabet cuts the per-column search by more than half and makes short keys fall quickly.

History and context§

Attributed to Count Gronsfeld in the seventeenth century, and popular precisely because a digit key is easy to remember and to dictate.

See also§

Vigenère · Beaufort · Variant Beaufort · Porta · Quagmire III (keyed alphabet) · Sum-clock (additive wheels)

Browse every cipher in the Polyalphabetic ciphers family, read the history of codebreaking, or return to the wiki main page.

Frequently asked questions§

How is the Gronsfeld broken?

Easier than Vigenere: the reduced alphabet cuts the per-column search by more than half and makes short keys fall quickly.

What key does the Gronsfeld use?

digits 0-9. The keyspace is unbounded in practice.

How much ciphertext do I need?

At least 24 characters for this solver to attempt it; short messages can be readable and still not be proof.

Can I break a Gronsfeld on this page?

Yes — the browser solver on this page handles it directly. Paste the ciphertext into the solver and press the button.

Categories: Polyalphabetic ciphers | Breakable in the browser | Cipher wiki
The facts in this article are generated from buttcrack's cipher registry when the site is built, so they describe the implementation you can run on this page. Registry-generated content is checked against the code; the history is editorial.

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It tries: Caesar, Atbash, ROT13, affine, Trithemius, rail fence, single-byte XOR, periodic Vigenère-family ciphers, autokey, selected encoding layers (such as Base64, hex, binary, decimal ASCII, Morse, and reverse), and—only with 60+ A–Z letters—statistical substitution.

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